Theories that account for renal medullary handling of solutes and water frequently neglect the role of the microcirculation. In contrast, studies shown that descending (DVR) and ascending vasa recta (AVR) have markedly different transport properties and that DVR express the aquaporin-1 water channel (AQP1) and a urea transporter (UT3). Our long-term objective is to determine the transport properties of these vessels and incorporate those findings into an integrated theory of salt and water handling by the renal medulla. The goals of the current proposal are the following. Aim 1: The role of AQP1 in DVR will be studied by comparing the conductances of the DVR wall to water transport in AQP1+/+ wildtype and AQP1 -/- knockout mice by in vitro microperfusion. Residual osmotic water permeability of AQP1 -/- will be examined for sensitivity to pCMBS and thiourea. The Arrhenius activation energy of remaining pathways will also be determined. The effectiveness of adenovirus mediated gene replacement of AQP1 into AQP1 -/- DVR will be assessed. Water handling by vasa recta of AQP1+/+ wildtype and AQP1 -/- knockout mice will be examined by measuring protein concentration and hydraulic pressure (Starling forces) in papillary vessels by micropuncture in vivo. The hypothesis that deletion of AQP1 will lead to water uptake by papillary DVR of -/- vs water efflux in +/+ controls will be tested. Aim 2: To followup studies that showed regulation of UT3 urea transporter mRNA by diuretic state, the long-term effect of hydropenia and water diuresis on urea permeability of the DVR wall will be examined. Short term modulation of urea and sodium transport by osmolality and vasopressin V1 or V2 receptor subtype agonists will also be studied. Aim 3: To characterize countercurrent exchange of middle and high molecular weight solutes, permselective properties of vasa recta will be examined by measuring molecular sieving of fluorescent dextrans and albumin in comparison to a high molecular weight, impermeant volume marker. The role of endothelial intracellular calcium concentration as a modulator of DVR transport properties will be examined in bradykinin treated vessels and in vessels from which albumin has been removed from the luminal or abluminal surfaces. Aim 4: Data from the above will be incorporated into a mathematical model of the medullary microcirculation. Previous models will be revised to enable exploration of the roles of the AQP1 water channel and UT3 urea transporter in the optimization of urinary concentrating ability. Solute and water generation rates will be assigned in the medullary interstitium and the resultant interstitial osmolalities determined for conditions in which AQP1 or UT3 have been deleted from DVR and/or red blood cells. Exchange of albumin and middle molecular weight solutes will also be simulated.